1. Field of the Invention
The present invention relates to a zoom lens, camera apparatus, information device having a photographing function and a mobile information terminal apparatus.
2. Description of the Related Art
Recently, in widely used digital cameras, high performance and small size have been further required and therefore a high-performance and small-size zoom lens which is installed therein as a photographing lens is also required.
In the zoom lens, in order to reduce the size, it is required to reduce an entire length (distance from a most object side lens surface to an imaging plane) in use, and it is important for reduction of size to reduce an entire length in a collapsed state by reducing a thickness of each lens group.
As the high performance of the zoom lens, a resolution corresponding to an image pickup device having at least 8 million pixels, more preferably 10 million pixels, further preferably 15 million pixels is required for an entire zooming range.
Furthermore, a wider field angle of a photographing lens is required by many users and a half field angle of the zoom lens at a wide angle end is preferably 38 degrees or more. The half field angle of 38 degrees corresponds to a focal length of 28 mm in a case of a silver-salt camera using a silver-salt film (so-called Leica film) having a width of 35 mm.
Furthermore, high magnification ratio is also required. The zoom lens having a focal length corresponding to of 28 to 200 mm in a 35 mm silver salt camera conversion (about 7.1 times) enables all of the general photographing. However, the magnification ratio of about 8 times is commonly used and higher magnification ratio is required, that is, the focal length corresponding to 28 to 300 mm in a 35 mm silver salt camera conversion (about 10.7 times) is required.
As the zoom lens for the digital camera, various types of the zoom lenses are used. However, lens group configuration including 5 lens groups or more is not suitable because it is difficult to achieve small size and small thickness of the entire lens system.
As a well known zoom lens having high magnification ratio or large diameter, the zoom lens including a first lens group having a positive focal length, a second lens group having a negative focal length, a third lens group having a positive focal length, and a fourth lens group having a positive focal length, which are disposed in order from an object side is disclosed in each of Japanese Patent Application Publication Nos. 2008-107559, 2008-112013, 2008-185782, 2008-203453 and 2008-145501.
In the zoom lens disclosed in the above documents, which has a four-lens group configuration including positive-negative-positive-positive lens groups, there is no zoom lens having all of a half field angle of 38 degrees or more, a magnification ratio of 8 times or more, and a resolution corresponding to 8 million pixels.
As the zoom lens suitable for a high magnification ratio, the zoom lens including a first lens group having a positive focal length, a second lens group having a negative focal length, a third lens group having a positive focal length and a fourth lens group having a positive focal length, which are disposed in order from an object side. When changing the magnification from a wide angle end to a telephoto end, an interval between the first lens group and the second lens group is increased, an interval between the second lens group and the third lens group is decreased and an interval between the third lens group and the fourth lens group is changed.
As the zoom lens of this type, the first lens group is reciprocated and moved in an arc which is convex at an image side when changing the magnification. In this type, if a movement amount of the second lens group which largely shares a function of the magnification operation in order to achieve the high magnification ratio is attempted to be largely ensured, an aperture stop provided at a vicinity of the third lens group is far from the first lens group even when the zoom lens is at the wide angle end. Accordingly, when the wide angle zoom lens is required, the first lens group is required to be large.
Therefore, in order to achieve a wide angle, high magnification and small size zoom lens, the first lens group is preferably moved so as to be positioned at the telephoto end to the position which is at the object side of the position at the wide angle end.
The entire length of the zoom lens at the wide angle end is reduced so as to be less than the entire length of the zoom lens at the telephoto end so that the size of the first lens is prevented from increasing and a sufficient wide angle can be achieved.
On the other hand, in a correction of the chromatic aberration which readily occurs with increasing the magnification ratio and the focal length, it is effective to use the lens having an anomalous dispersion property, as well known.
As the zoom lens including a first lens group having a positive focal length, a second lens group having a negative focal length, a third lens group having a positive focal length, and a fourth lens group having a positive focal length, which are disposed in order from an object side, and in which an interval between the first lens group and the second lens group is increased, an interval between the second lens group and the third lens group is decreased, and an interval between the third lens group and the fourth lens group is changed when changing the magnification, the zoom lens using an anomalous dispersion property is disclosed in Japanese Patent Application Publication Nos. H08-248317, and 2008-026837 and Japanese Patent Nos. 3391342 and 4405747.
In these Documents, in the zoom lens disclosed in Japanese Patent Application Publication No. H08-248317, the first lens group is fixed when changing the magnification and half field angle at the wide angle end is 25 degrees and a wide angle end is not sufficient.
The zoom lens disclosed in Japanese Patent No. 3391342 includes a four-lens group configuration having positive-negative-positive-positive lens groups (see Examples 1, 2 and 6) and the zoom lens has a field angle of about 29 to 32 degrees at the wide angle end so that the wide angle is not sufficiently achieved.
The zoom lens disclosed in Japanese Patent No. 4405747 has a wide angle, that is, a half field angle of about 37 degrees at the wide angle end. However, the lens configuration includes many lenses, that is, 14 lenses and therefore it is difficult to achieve the reduced entire length in a collapsed state and low cost.
The zoom lens disclosed in Japanese Patent Application Publication No. 2008-026837 has a comparatively small number of lenses, that is, 9 to 11 lenses so that wide angle and high magnification is achieved with a simple configuration. However, there is room for improvement in the small size because the entire length at the telephoto end is slightly large.
For example, the zoom lens disclosed in Japanese Patent Application Publication No. 2008-107559 has a wide field angle, that is, a half field angle of 40 degrees at the wide angle end, but the magnification ratio is about 5 times. The zoom lens disclosed in Japanese Patent Application Publication No. 2008-112013 has a wide angle, that is, a half field angle of substantially 40 degrees at the wide angle end and a large magnification ratio of 9 times. However, the distortion at the wide angle end is 10% or more and therefore there is room for improvement in performance.
Japanese Patent Application Publication No. 2008-185782 discloses, as a specific example, the zoom lens having a magnification ratio of about 9.5 times, but a half field angle is about 32 degrees at the wide angle end.
The zoom lens disclosed in Japanese Patent Application Publication Nos.2008-203453 and 2008-145501 has a good performance and a wide angle but there is room for improvement in the magnification ratio.
As a general method for reducing a size of the zoom lens, it is known to reduce a number of lenses by increasing a power of each lens group. However, when simply using this method, it is difficult to correct various aberrations and decentering sensitivity of each lens groups (degradation degree of an imaging performance by decentering lens) is increased so that it is difficult to maintain a high optical performance.
If the wide field angle is tried to be achieved while performing this method, it becomes further difficult to correct various aberrations while suppressing increase in size of the entire lens system due to increase in height of off-axis light fluxes or increase of the distortion.